1. Field of the Invention
The present invention relates to a focus error detection device, and more specifically to a focus error detection device for an optical recording/playing back system.
2. Description of Background Information
In a system for optically recording or playing back information such as a compact disc player system or a video disc player system, it is general to provide a focus servo control system by which a record or a read out light beam is correctly focused on a record medium. Conventional focus error detection devices used in such systems are constructed such that a reflection beam is directed through a cylindrical lens to a light detector made up of a plurality of light receiving elements which are arranged symmetrically with respect to the center of the light detector. The focus error signal is derived by the subtraction between two sum signals each of which being obtained by the summation of output signals of two of the light receiving elements which are arranged symmetrically with respect to the center of the light detector. However, the drawback was that an off-set of the focus error signal is inevitably generated when the direction of the light beam is shifted tangentially or normally to the recording tracks for the purpose of tracking control or jitter correction.